I have recently read about COUNTD will not work if the connection type is LIVE, it should work only for EXTRACT, But this article and the questions was posted on 2016 almost 3 years back...so I am sure it is fixed now...

You are correct that the Jet driver on a live connection has limitations. One of those is COUNTD, MEDIAN is another.

From at least version 9.0 (perhaps earlier as suggested?) Tableau stopped using the Jet driver and implemented its own. There were a few reasons for this but, speed, functionality and compatibility with Mac were big ones. You can still dial back to the legacy connection if you desire.